What is the significance of measuring preductal and postductal oxygen saturation in newborns, particularly in relation to congenital heart defects?

Understanding Preductal and Postductal Oxygen Saturation in Newborns

Core Anatomical Concept

Preductal saturation is measured from the right upper extremity (wrist or medial palm) and reflects oxygenation before blood passes through the ductus arteriosus, while postductal saturation is measured from either lower extremity and reflects oxygenation after the ductus arteriosus. 1, 2

• The ductus arteriosus is a fetal vessel connecting the pulmonary artery to the descending aorta that normally closes after birth 1
• In certain pathological conditions, blood can shunt right-to-left through a patent ductus arteriosus, causing deoxygenated blood from the pulmonary artery to enter the descending aorta 3
• This creates a measurable difference between upper extremity (preductal) and lower extremity (postductal) oxygen saturations 1, 4

Clinical Significance: What These Measurements Tell You

Detection of Critical Congenital Heart Disease (CCHD)

The primary purpose of measuring both preductal and postductal saturations is to screen for critical congenital heart defects, particularly ductal-dependent lesions. 2, 3

• Higher preductal saturation compared to postductal saturation indicates right-to-left shunting through the ductus arteriosus, suggesting right ventricular outflow obstruction or pulmonary atresia 3
• This pattern is characteristic of ductal-dependent pulmonary flow lesions including pulmonary atresia, critical pulmonary stenosis, or severe tetralogy of Fallot 3
• The difference occurs because relatively well-oxygenated blood reaches the right arm before mixing with deoxygenated blood shunting through the ductus to the lower body 3, 4

Screening Protocol and Thresholds

CCHD screening should be performed at approximately 24 hours of life using simultaneous pre- and postductal measurements. 2, 5

Abnormal screening is defined as: 2, 5

• Oxygen saturation <95% in both preductal AND postductal sites, OR

• A >3% difference between preductal and postductal measurements, OR

• Any single measurement <90%

• If initial screening is abnormal, one retest should be performed 5

• Screening must be performed on room air to avoid false negatives 5

• Failed screening after retest requires immediate echocardiography 2, 5

Normal Values at Sea Level

In healthy term newborns at approximately 24 hours after birth, mean preductal saturation is 98.29% (median 98%) and mean postductal saturation is 98.57% (median 99%). 6

• Postductal saturation is equal to or slightly higher than preductal saturation in 78% of healthy newborns 6
• The mean difference is only -0.29% (postductal slightly higher) 6
• At high altitude (2820 meters), mean preductal saturation drops to 92.76% and postductal to 93.76%, requiring adjusted cutoff points 7

Guiding Neonatal Resuscitation

During neonatal resuscitation, oxygen therapy should be titrated to achieve preductal saturations matching the interquartile range of healthy term infants. 1, 2

• Pulse oximetry should be used when resuscitation is anticipated, when positive pressure ventilation is administered for more than a few breaths, or when supplementary oxygen is given 1
• The probe must be attached to a preductal location (right upper extremity) to appropriately compare to published normative data 1, 2
• Term/late-preterm (≥35 weeks) newborns should start resuscitation with 21% oxygen (room air) 2
• Preterm (<35 weeks) newborns should start with 21-30% oxygen 2
• 100% oxygen is NOT recommended for initial resuscitation of term infants (Class 3: Harm) 2
• If bradycardia (HR <60/min) persists after 90 seconds with lower oxygen concentration, increase to 100% until heart rate recovers 1

Identifying Other Critical Conditions

Persistent Pulmonary Hypertension of the Newborn (PPHN)

In newborn septic shock with PPHN, a difference in preductal and postductal oxygen saturation <5% is a therapeutic endpoint. 1

• PPHN causes right ventricular failure with right-to-left shunting at atrial/ductal levels causing cyanosis 1
• Target 95% arterial oxygen saturation while maintaining the pre-post ductal difference <5% 1
• Monitor both preductal and postductal pulse oximetry continuously 1

Congenital Diaphragmatic Hernia (CDH)

In severe CDH with suprasystemic pulmonary hypertension, preductal and postductal arterial blood gases differ dramatically due to right-to-left ductal shunting. 4

• Preductal pH averages 7.35 vs postductal 7.30 4
• Preductal PaO2 averages 99 mmHg vs postductal 46 mmHg 4
• Preductal oxygen saturation averages 95.6% vs postductal 85.2% 4
• Ventilator management should target preductal arterial blood gas measurements to avoid over-ventilation based on falsely low postductal values 4

Technical Considerations for Accurate Measurement

Modern neonatal pulse oximeters provide reliable readings within 1-2 minutes after birth when there is sufficient cardiac output and skin blood flow. 1, 2

• Attach the probe to the baby before connecting to the instrument for most rapid signal acquisition 1
• For screening purposes, measurements should be simultaneous rather than sequential 2, 6
• Postductal measurements can be obtained from either lower extremity 8, 9

Critical Pitfalls to Avoid

False Negatives

CCHD cannot be ruled out based solely on normal screening, as pulse oximetry sensitivity for CCHD is only 50-76%. 2, 5

• Some ductal-dependent lesions (like coarctation of the aorta) may not cause hypoxemia until after ductal closure 5
• One study missed 1 neonate with aortic isthmus stenosis and 1 with pulmonary stenosis despite saturations >95% 9
• Clinical examination remains essential alongside pulse oximetry screening 9

False Positives Are Clinically Valuable

Most "false positive" screens actually identify clinically important non-cardiac conditions including persistent pulmonary hypertension, pneumonia, and sepsis. 5, 8, 9

• In one study, 5 of 7 infants with saturations <95% but no CHD had persistent pulmonary hypertension 8
• Another study found neonatal infections in 7 of 18 newborns with abnormal pulse oximetry 9
• The positive predictive value for CHD is approximately 50%, but overall clinical utility is much higher 9

Distinguishing Ductal-Dependent Lesions from Other Shock States

Any newborn with shock and hepatomegaly, cyanosis, cardiac murmur, or differential upper/lower extremity blood pressures or pulses should be started on prostaglandin E1 infusion until complex congenital heart disease is ruled out by echocardiography. 1

• This applies even before definitive diagnosis 1, 3
• Target oxygen saturation between 75-85% in suspected ductal-dependent lesions to avoid excessive pulmonary blood flow 3

Performance Characteristics of Screening

Implementation of CCHD screening has decreased early infant deaths from CCHD by 33%. 5

• Sensitivity ranges from 50-82% depending on the study 2, 9
• Specificity is consistently 99.9% 9
• Positive predictive value is approximately 50% for cardiac disease specifically 9
• Negative predictive value is 99.9% 9
• Early detection reduces emergency hospitalizations and improves treatment outcomes 5, 8